Device for pressurizing a unified two-liquid propulsion subsystem geostationary satellites

ABSTRACT

The invention relates to a device for pressurizing a standardized two-liquid propulsion system of a geostationary satellite comprising a high pressure helium storage unit (10) having at least one helium storage tank, a regulating unit (11) and a pressurizing unit for propellant tanks (17, 18) interconnected by a pressurizing circuit, wherein the regulating unit is an electronic regulating unit comprising at least one electrovalve (51), positioned at a location of the pressurizing circuit and controlled by a processing and control circuit (54), which receives informations from pressure sensors (43, 47, 48) associated with the different tanks, said processing and control circuit permitting a constant pressure pressurization of the propellant tanks (17, 18) by flow lines including a combination of pyrotechnic valves, filter elements and calibrated leak elements during the satellite transfer and placing in position phase and a measurement of the residual propellant quantity during the orbital life phase of the satellite so as to predict satellite End of Life to within 2 months out of a 15 year mission.

DESCRIPTION

1. Technical Field

The present invention relates to a device for pressurizing astandardized two-liquid propulsion subsystem of a geostationarysatellite.

2. Prior Art

Telecommunications satellites are placed on an orbit in the earth'sequatorial plane at approximately 36,000 kilometres from the earth. Thiscircular orbit has the special feature that any satellite object on thisorbit has a 24 hour rotation period around the earth. Thus, such anobject appears stationary above the earth for an observer on earth. Thispreferred orbit enables telecommunications satellites to best fulfiltheir communication relay function between several points on earth.

In order to inject a satellite into its final orbit and keep it thereduring its useful life, propulsion means must be provided. Standardizedsubsystems are now used in a general manner. A tendency occurring atpresent consists of providing a standardized propulsion subsystem, whichinjects the satellite into its final orbit from its initial or transferorbit and then keeps the satellite in position.

In such a standardized two-liquid propulsion subsystem, the liquidbi-propellant engine or motor is supplied from the same tanks andprimary circuits as the other engines necessary for attitude and orbitcontrol. As a result the transfer phase must take place with thepropulsion system under a regulated helium pressure of approximately 18bars and which is stored in high pressure tanks and once placed inposition, the apogee engine and helium source are isolated from theremainder of the subsystem, which then operates at a decreasing pressureas the propellants are consumed in the main tanks. Generally, use ismade of two propellants, e.g. monomethyl hydrazine (MMH) as the fuel andnitrogen peroxide (N₂ O₄) as the oxidizer.

Up to now most geostationary telecommunications satellites use such astandardized two-liquid propulsion subsystem.

The user to whom a precise position on the geostationary orbit has beenallocated, only wishes to keep on this position satellites which areeffective for his particular communications traffic system. Therefore,he regularly replaces any satellite which has become obselescent by anew satellite. The obselescent satellite must then be sent into an orbitwhere it will not interfere with the new satellites or those around it,when its obsolescence has been established, in order to optimize theprofitability of the system installed.

The most frequent reason for ending the life of the satellite is that ithas consumed all the propellants maintaining it in position and itspointing towards the earth.

During the satellite life, a significant part of the propellant masscarried is burned in the apogee engine, whose aim is to increase thespeed vector of the satellite in order to pass it from an elongated,elliptical orbit known as the transfer orbit to the circular orbit at analtitude of 36,000 km.

Once placed in its orbital position, the satellite is subject todisturbance forces (lunar/solar gravity, solar radiation pressure,etc.), which tend to make it describe cyclic movements with a decreasingamplitude around the initial position. As the ground stations arepointed towards a fixed point in the sky, in order to avoidoverequipping the ground stations with satellite tracking means, and inorder to keep the satellite in the allocated orbital position, it isnecessary to periodically intervene in order to oppose interferingmovements of the satellite, by periodically burning a quantity ofpropellants in the orbit control motors. The satellite is also exposedround its centre of gravity to disturbance torques, which would lead tothe depointing thereof. Thus, a quantity of propellants is alsoperiodically burned in the attitude control motors.

The user needs to know the satellite obsolescence date sufficiently wellin advance in order to be able to initiate the activities necessary forits replacement. Thus, the satellite operator must be able to reliablypredict the end of propellant consumption, so as to be able to activatethe orbit ejection operation. If the information system is not precise,the greater the imprecision of the means the greater the satellite useloss and marketing time loss.

Various prior art devices aim at solving these problems. An articleentitled "Low-gravity propellant gauging system for accurate predictionsof spacecraft end-of-life" by M. V. Chobotov and G. P. Purohit (Journalof Spacecraft and Rockets, vol. 30, No. 1, January/ February 1993)describes a system for evaluating the propellant in a low gravityenvironment and predict the end of life of a satellite to withinplus/minus two months, at half life for a nominal mission of fifteenyears in the case of geosynchronous satellites. The system comprises twopressure sensors, i.e. one for each tank, and an interconnection lockingvalve between the two tanks, which are respectively the propellant andpressurized gas (helium) tanks in the satellite propulsion system. Inorder to carry out a propellant measurement, the propellant tank isrepressurized by a brief opening of the interconnection valve betweenthe pressurized tank at a relatively higher pressure and the propellanttank. The interconnection valve is closed again prior to the equallingout of the pressure between the two tanks in order to allow theperformance of multiple measurements during the satellite mission. Themeasurements of the pressures and temperatures of the tanks before andafter repressurization make it possible to determine the residual gasvolume in the propellant tank by calculation according to the law ofgases. On the basis of the knowledge of this volume and the total volumeof the propellant tanks, it is possible to deduce the liquid volumepresent and consequently the available propellant mass.

German patent application DE-8 806 777 describes a zero-gravitymeasurement device for the liquid quantity, particularly the propellantquantity in tanks of satellites, in which a propellant tank is connectedto a compressed gas tank by means of a pressurized gas supply line, inwhich is installed a measuring device giving the gas quantitytransferred during the, implementation of the device. A shutoff orinterconnection valve makes it possible to maintain a pressure in thetank containing the compressed gas higher than that of the propellanttank. A pressure sensor also determines the pressure of the gascontained in the propellant tank.

French patent application 91 15441 of 12.12.1991 describes a device forthe periodic measurement of the residual liquid volume in a pressurizedsealed tank containing a liquid, which is progressively drawn off, and agas slightly soluble therein, which comprises a pressurized gas source,a reducing regulator supplying the tank and constructed so as topermanently prevent the pressure in the tank from dropping below a givenvalue P, means permitting the admission of gas under a pressure P+ΔPinto the tank through a constriction and means for measuring the passagetime of the internal pressure from P to P+ΔP.

French patent application 2 629 913 describes a process for evaluatingthe residual fuel quantity in the tank of a spacecraft placed in orbit,according to which determination takes place of the gas volume in thefuel tank at a given instant and this quantity is deducted from thetotal volume of said tank. A first pressure measurement P1, P2 takesplace at the outlet of the pressurization tank and the inlet of the fueltank and a first temperature measurement T at a predetermined timeduring the space mission, followed by a second pressure measurement P'and a second temperature measurement T' after directly linking theoutlet of the pressurized gas tank and the inlet of the fuel tank, themeasurements of pressure P1, P2, P' and temperature T, T' serving todetermine the gas volume in the fuel tank. The system permitting theperformance of this process comprises:

a line directly connecting the pressurization gas tank and the inlet ofthe fuel tank, said line containing a venturi,

a first valve responding to a control signal to permit the introductionof the pressurization gas into the line at the start of the spacemission,

a second valve responding to a control signal for closing thecommunication between the line and the fuel tank at a predeterminedinstant during the space mission,

a first pressure transducer connected in order to measure the pressureP1, ΔP1 upstream of the first valve and

a second pressure transducer for measuring the pressure P2, ΔP2downstream of the second valve.

In the prior art devices, pressure regulation during the transfer phasestill takes place by means of a mechanical pressure reducing regulator(sometimes redundant in series).

However, the measurement of residual propellants during the orbital lifephase takes place by means of independent valves connected in parallelto the main, regulated pressurization line. The pressurization of thetwo-liquid propulsion subsystem of the satellite and the measurement ofthe quantity of residual propellants take place during said orbital lifephase by a repressurization (or reinflation) method.

The object of the invention is not the measuring method used, which isthat described in French patent application FR-A-2 629 913 (Gindre) andin the article entitled "Low-gravity propellant gauging system foraccurate predictions for spacecraft end-of-life", analyzed hereinbefore.

The object of the invention is in fact to ensure by a single (redundant)subassembly of valves and calibrated orifices, the two regulatedpressurization functions during the transfer phase and the placing inposition of the satellite and reinflation during the orbital life phase.

DESCRIPTION OF THE INVENTION

The present invention proposes a device for pressurizing a standardizedtwo-liquid propulsion system of a geostationary satellite comprising ahigh pressure helium storage unit having at least one helium storagetank, a regulating unit and a pressurizing unit for propellant tanksinterconnected by a pressurizing circuit, characterized in that theregulating unit is an electronic regulating unit comprising at least oneelectrovalve, positioned at a location of the pressurizing circuit andcontrolled by a processing and control circuit, which receivesinformations from pressure sensors associated with the different tanks,said processing and control circuit permitting a constant pressurepressurization of the propellant tanks during the satellite transfer andplacing in position phase and a measurement of the residual propellantquantity during the orbital life phase of the satellite.

Advantageously, the helium storage tanks communicate with the fuel tankand the oxidizer tank by means of several lines. In a first line isinserted a first pyrotechnic valve, a first filter, a second and thirdpyrotechnic valves in parallel, each being placed in series with acalibrated leak element. In a second and a third lines are inserted acheck valve, a fourth or sixth pyrotechnic valve and a fifth or seventhpyrotechnic valve separated by a second or third filter.

The helium storage circuit is completed by a ground filling and drainingvalve, a test connector and a pressure system. The pressurization unitis completed by test connectors and pressure sensors.

In an embodiment the electrovalves are respectively placed between thesecond (and third) pyrotechnic valve and the calibrated leak elementcorresponding thereto between the second filter and the fifthpyrotechnic valve and between the third filter and the seventhpyrotechnic valve. The processing and control circuit receivesinformations from pressure sensors associated with the helium tanks andpressure sensors associated with the propellant tanks.

In another embodiment the device according to the invention has at leasttwo fuel tanks and at least two oxidizer tanks, the elements of thesecond and third lines associated with the first tanks then beingduplicated. Therefore the pyrotechnic valves and branch elements,illustrated in FIG. 4 with an apostrophe are relative to a configurationof the propulsive subsystem having more than two propellant tanks.

The device according to the invention, in its constant pressurizationfunction, can be likened to an electronic regulator. It in particularhas the advantage of permitting an adjustment or modification in flightof the regulated pressure thresholds, which is impossible with amechanical regulator

During the orbital life phase, the electrovalves ensure the isolation orseparation of the high pressure helium part in the propellant tanks witha requisite sealing level. The measurement of the residual propellantsin each tank can take place at regular intervals during the orbitallife. By an opening/closing of electrovalves, it consists ofreintroducing helium into the propellant tanks. The knowledge of thepressure variations supplied by the sensors then provides information onthe free volume and consequently that of liquid present in thepropellant tanks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates the device according to theinvention.

FIGS. 2 and 3 show curves illustrating the pressure evolution in thepropellant tanks as a function of time.

FIG. 4 relates to the liquid part of the propulsive subsystem locateddownstream of the device according to the invention, which is itselflocated in the gas pressurization part.

DETAILED DESCRIPTION OF EMBODIMENTS

As shown in continuous line form in FIG. 1, the device according to theinvention comprises:

a high pressure helium storage unit 10 in particular comprising thehelium storage tanks 13, 14, 15 and 16,

an electronic regulating unit 11,

a pressurization unit 12 more particularly comprising two propellanttanks, namely a fuel tank 17 and an oxidizer tank 18.

The helium storage tanks 13, 14, 15 and 16 (here four in parallel)communicate with the fuel tank 17 and oxidizer tank 18 by lines 20, 21and 22.

In the first line 20 is inserted a first pyrotechnic valve 23, a firstfilter 24, a second and a third pyrotechnic valves 25 and 26 inparallel, each being arranged in series with a calibrated leak element27 and 28.

In each of the second and third lines 21 and 22 are inserted a checkvalve 30 and 31, a fourth pyrotechnic valve and a fifth pyrotechnicvalve 32 and 36 separated by a second filter 38 (or a sixth and aseventh pyrotechnic valves 33 and 37 separated by a third filter 39).

The helium storage circuit 10 is completed by a ground filling anddraining valve 41, a test connector 42 and a pressure sensor 43. Thepressurization unit 12 is completed by test connectors 45, 46 and 55 andpressure sensors 47 and 48.

According to the invention, introduction takes place of severalelectrovalves 50, 51, 52 and 53 controlled by a processing and controlcircuit 54, at different locations of said circuit, respectively betweenthe second 25 and third 26 pyrotechnic valve and the correspondingcalibrated leak element 27 or 28, between the second filter 38 and thefifth pyrotechnic valve 36 and between the third filter 39 and theseventh pyrotechnic valve 37. The processing and control circuit 54receives informations from the pressure sensors 43 associated with thehelium tanks 13, 14, 15 and 16 and pressure sensors 47 and 48 associatedwith the propellant tanks 17 and 18.

However, it is pointed out that the invention also functions with asmaller number of valves (the valve 50 is here to ensure redundancy onlyand is not essential to the operation of the device). Moreover, theelectrovalves 52 and 53 can function independently and consequentlypermit an individual and independent measurement of liquid quantitiespresent in the tanks.

The filling valve 41 makes it possible to fill the helium tanks 13, 14,15 and 16 on the ground at 300 bars, the first pyrotechnic valve 23normally being closed, the remainder of the circuit remaining at lowpressure. This valve is opened following the separation of the satellitefrom the launcher. The check valves 30 and 31 make it possible to avoida mixture of the two propellants which would be explosive. The filters24, 38 and 39 located at the outlet of the pyrotechnic valves protectthe downstream circuits.

The fifth and seventh pyrotechnic valves 36 and 37 normally remain opento the end of the satellite life. They are only used in the case of aleak and an explosion risk, their firing making it possible to isolatethe propellant tanks 17, 18 from the helium tanks 13, 14, 15 and 16.

It should be noted that the pyrotechnic valves, their number and theirlocation are in particular dictated by safety considerations and theneed to protect the firing site authorities on the ground.

The object of the device according to the invention is to ensure both aconstant pressure pressurization function of the propellant tanks duringthe transfer and placing in position phase of the satellite and arepressurization or reinflation function of the propellant tankspermitting a measurement of residual propellant quantities during theorbital life phase.

The processing and control circuit 54 sends open/close instructions tothe electrovalves in the pressurization circuit of the propellant tanks.Thus, during the satellite transfer and placing in position phase,following the bringing of the subsystem into configuration, thepressurization and maintenance thereof take place by means ofelectrovalves, whose open/close instructions are processed by theprocessing and control circuit by comparing data supplied by thepressure sensors with preset regulating thresholds, which can beoptionally modified remotely.

The curves giving the propellant pressure PE as a function of time tshown in FIGS. 2 and 3, illustrate the evolution of the controlfrequency of the regulating electrovalve 51 as a function of the timeduring the transfer phase and apogee manoeuvres, the gas volume in thepropellant tanks being 100 litres in FIG. 2 and 400 litres in FIG. 3.This operating example is considered for regulating thresholdsrespectively fixed at 17.9 and 18.1.

Therefore the device according to the invention has two operatingphases.

1. During the transfer and placing in position sequence

Following the placing in configuration of the subsystem, i.e. afteropening the normally closed pyrotechnic shutoff valves (23, 26, 32, 33),the pressurization and the maintaining thereof throughout the transferphase take place by means of electrovalves, whose open/closeinstructions are produced by the processing and control electronics onthe basis of a comparison of data supplied by the pressure sensors withpreregulated regulation thresholds. A failure of the nominal path orchannel leads to the closing of the pyrotechnical valve (29) and the notnecessarily simultaneous opening of the redundant channel (25).

2. In orbit

The electrovalves ensure the isolation of the high pressure helium part(in which there are only about 60 bars after the putting into position)from the propellant tanks with the requisite sealing level. Themeasurement of the residual propellants in each tank can take place atregular intervals during the orbital life. It consists of reintroducinghelium into the propellant tanks by opening/closing electrovalves.Information on pressure variations supplied by the sensors then makes itpossible to establish the free volume and consequently the liquid volumepresent in the propellant tanks.

The failure or sealing deficiency of valves, noted from the pressuresensors, leads to the closing of the pyrotechnic valves, which thentotally isolates the helium circuit and there is a restoration to theconfiguration now existing on all subsystems evolving in the blow-downmode (decreasing pressure) during the orbital life.

In FIG. 1, completed with mixed line parts, is represented a variant ofthe device according to the invention with on this occasion several,e.g. two fuel tanks and several, e.g. two oxidizer tanks.

Elements identical to those of FIG. 1 described hereinbefore retain thesame references. The new elements playing an equivalent part assume thesame reference as the corresponding element, but followed by anapostrophe.

Thus, FIG. 4 shows the three units 10, 11 and 12 also shown in FIG. 1.However, the liquid apogee motor 60, the first engine group 61 and thesecond redundant engine group 62 are also shown. FIG. 4 also shows thedifferent elements necessary for the supply of said motors and engines,namely:

pyrotechnic valves 65, 66, 67 and 68,

shutoff valves 70, 71, 72, 73, 74 and 75,

filling valves 76, 77, 78 and 79,

test connectors 80, 81, 82, 83,

filters 84, 85, 86, 87, 88 and 89,

calibrated leak elements 90, 91, 92, 93, 94 and 95,

pressure sensors 96, 97, 98 and 99.

In an operational example of the variant of the device according to theinvention illustrated in FIGS. 1 (total) and 4, the following stagesoccur.

1. Initial stage

The helium pressure upstream of the first, normally closed pyrotechnicvalve 23 is 300 bars. The circuit downstream of said valve and up to theshutoff valves 70, 71, 72 and 73, which are in the closed position, isunder a helium pressure of approximately 4 bars. The tanks are 98%filled with propellants (for safety reasons on the ground in the case ofa temperature rise). The remainder of the circuit is also pressurizedunder 4 bars helium. The shutoff valves 74 and 75, which in flight makeit possible to select the functional branch or redundant branch, areclosed.

2. Placing in configuration sequence

Following launcher separation, the following operations occur:

venting the lines or pipes,

opening the pyrotechnics 36 and 37,

opening the shutoff valves 74 and 75,

opening the apogee motor valves 60 and one or both attitude engines forabout 30 seconds.

The satisfactory performance of these operations is followed by theplacing under pressure of lines and equipment:

filling lines with propellants (priming),

opening the shutoff valves 72 and 73 (a single tank of each propellant),

the pressure of the lines must be at approximately 4 bars (initial tankpressure):

pressurization of the subsystem,

opening the shutoff valves 70 and 71,

opening the pyrotechnic valves 32, 32', 33 and 33' (said valves beingused for isolating the propellants from one another up to this time,after which the electrovalves 52, 52', 53 and 53'and the check valves 30and 31 will fulfil this function),

opening the valves 52, 52', 53 and 53',

opening the pyrotechnic valve 23.

At this instant, the pressurization device is ready to operate.

There is a "regulated" pressure rise by starting the pressurizationsequence. The comparison of the data from the sensors 47, 47', 48 and48' with the reference threshold displayed in the processing and controlelectronics 54 (typically 18 bars) controls the opening and closing ofthe electrovalve 51. (which is coupled to a calibrated orifice). In thecase of a failure, passage takes place to the redundant branch (closingthe pyrotechnic valve 26, opening the pyrotechnic valve 25 andregulation by the electrovalve 50). The system is then operational andas from then on the engines can be used.

3. Geostationary orbit transfer phase

For the operation of the liquid apogee motor 60 during apogeemanoeuvres, this phase requires that the pressure is regulated in thetanks. When a flow occurs, this regulation is ensured by theelectrovalve 51 or electrovalve 50 on orders provided by the processingand control electronics 54. Between each apogee manoeuvre, theelectrovalves 52, 52', 53 and 53' are closed again (although this is notstrictly necessary).

4. Flight phase

At the end of the transfer phase on existing satellites, the pyrotechnicvalves 36, 36', 37 and 37' are closed in order to definitively isolatethe helium. As from then on the pressure will decrease in the tanks asthe propellants are being consumed. With the device according to theinvention, said valves are only used in the case of a failure and leadto the above configuration.

By programmed opening of e.g. electrovalve 51 and electrovalve 52', itis possible to reintroduce at regular intervals during the orbital life,helium into the tank 17'. The measurement of the pressure variations bythe sensors 43 and 47' then gives information on the liquid quantitypresent in said tank. Symmetrical action takes place for the othertanks.

The same procedure can be applied to a subsystem having two tanks, asshown in continuous line form in FIG. 1.

We claim:
 1. Device for pressurizing a standardized two-liquidpropulsion system of a geostationary satellite comprising a highpressure helium storage unit having at least one helium storage tank; apressurizing unit including propellant tanks; an electronic regulatingunit interconnected to the propellant tanks by a pressurizing circuitand including at least one electrovalve coupled with a calibratedorifice, the regulating unit being positioned such that the electrovalveand calibrated orifice are between the high pressure unit and thepropellant tanks; and a processing and control circuit controlling theelectrovalve, the processing and control circuit receiving informationfrom pressure sensors associated with the helium storage tank and thepropellant tanks and providing a regulated pressure in the propellanttanks during a transfer orbit of the satellite and repressurization ofthe propellant tanks at any time during an orbit life of the satellite.2. Device according to claim 1, wherein the propellant tanks include atleast one fuel tank and an oxidizer tank.
 3. Device according to claim 1wherein the processing and control unit is adapted to regulate, duringthe transfer orbit, pressure level in the propellant tanks responsive tosignals sent from earth.
 4. Device according to claim 1 wherein therepressurization maintains a pressure difference between propellanttanks.
 5. Device according to claim 1 wherein the repressurizationpermits measurement of residual liquid remaining in the propellanttanks.
 6. Device according to claim 1 wherein the repressurization isachieved by causing a predetermined pressure increase in the propellanttanks.
 7. Device according to claim 1 wherein the repressurization isachieved by opening the electrovalve at regular intervals.
 8. Deviceaccording to claim 1, wherein the helium storage unit is completed by aground filling and draining valve, a test connector and a pressuresensor.
 9. Device according to claim 2 wherein the repressurizationmaintains a pressure difference between the oxidizer tank and the fueltank so as to optimize propulsion system performance.
 10. Deviceaccording to claim 2, wherein the helium storage tanks communicate withthe first fuel tank and the first oxidizer tank by several lines, thatin a first line is inserted a first pyrotechnic valve, a first filter, asecond and a third pyrotechnic valve in parallel, each being placed inseries with a calibrated leak element and in that in each of the secondand third lines are inserted a check valve, a fourth or sixthpyrotechnic valve and a fifth or seventh pyrotechnic valve separated bya second or third filter.
 11. Device according to claim 10, wherein thepressurization unit is completed by test connectors and pressuresensors.
 12. Device according to claim 10, which comprises at least twofuel tanks and at least two oxidizer tanks, the elements of the secondand third lines associated with the first fuel tank and first oxidizertank then being duplicated in other lines for the other fuel tank(s) andoxidizer tank(s).
 13. Device according to claim 10, wherein a firstelectrovalve is placed between the third pyrotechnic valve and thecorresponding calibrated leak element and that two second electrovalvesare respectively placed between the second filter and the fifthpyrotechnic valve and between the third filter and the seventhpyrotechnic valve.
 14. Device according to claim 13, which comprises aredundant electrovalve placed between the second pyrotechnic valve andthe corresponding calibrated leak element.
 15. Device according to claim13, wherein the two second electrovalves operate separately and thuspermit an individual and independent measurement of liquid quantitiespresent in the tanks.